The present invention is directed to detecting and interrupting ground faults in circuits having a grounded load. Specifically, a ground fault circuit interrupter (GFCI) is provided that differentiates between ground faults and leakage and electromagnetic interference (EMI) induced currents generated in the load circuit and returned to the source through ground.
GFCI devices are intended to detect and interrupt potentially harmful ground fault currents. GFCI devices work well for this purpose unless electromagnetic interference or voltage transient leakages also set off the GFCI device. In these cases, the interference or transients cause a current sensor to detect in the phase lines a current that reaches or exceeds the trip setting of the ground fault interrupter, causing it to falsely activate in the absence of an actual fault.
In certain applications (such as aircraft devices and appliances) a secondary ground wire provides a path to ground for EMI or transient leakage currents. This path reduces the risk that such transients may pose to aircraft passengers and crew. The threshold for such a GFCI circuit has to be set so high to avoid false trips that a person in direct contact with such equipment could receive a fatal electrical shock. Trip voltages over 8 Milliamps are considered fatal. Typical transient voltages found in IFE applications exceed 20 Milliamps.
Underwriters"" Laboratories has studied the ground fault phenomenon and has issued UL 943, Category A as the standard for GFCI devices that protect human life for 50/60 cycle AC power systems. According to this standard, the minimum level for a dangerous ground fault current is 6 milliamps. The ability to distinguish leakage currents from ground fault currents in these applications is paramount. Electrical systems where individual equipment is required to have an attachment to system ground (such as aircraft devices and appliances) are inherently more difficult to monitor.
The concept of a ground fault circuit interrupter using a current transformer is not new. Such a transformer detects the current differential between the line and neutral wires. This detector may be used in both single phase and three phase AC power systems. Any leakage current, ground fault current or the sum thereof is interpreted as a ground fault occurrence.
U.S. Pat. No. 5,793,587 (Boteler) discloses a dual trip level ground fault interrupter having an equipment current transformer and a personnel current transformer. The personnel transformer is more sensitive to fault currents than the equipment transformer. An equipment ground conductor passes through the personal transformer but not through the equipment transformer. While the differential current transformer responds to any leakage current not returning on the ground conductor, the electrical circuit for the load is ungrounded. The Boteler configuration is thus inapplicable for devices having a grounded load (such as aircraft devices and appliances) for preventing electrical shock.
Thus, there exists in the art the need for a ground fault circuit interrupter that is sensitive to ground faults but nonresponsive to leakage and electromagnetic interference currents in systems having a grounded load.
The present invention is directed to an apparatus and method for detecting a ground fault in a grounded load circuit having a floating ground and powered through at least one line conductor and a neutral conductor. The system comprises a first load terminal for receiving a conductor connected to the floating ground, a second load terminal for receiving the neutral conductor, and a set of third load terminals for receiving at least one line conductor. A first source terminal is provided for receiving a conductor connected to absolute ground, a second source terminal is provided for receiving a conductor connected to the neutral of the power source, and a set of third source terminals are provided for receiving a set of source conductors connected to each source conductor.
A current sensor senses the current in a set of conductors that connect, respectively, the first load terminal and the first source terminal, the second load terminal and second source terminal, and the third load terminals and set of third source terminals.
The present invention may be particularly useful as a resettable ground fault responsive interrupt circuit for a 400 Hz aircraft electrical system. Such a system may comprise an AC input from the aircraft electrical system and a floating ground conductor for connection to the floating ground of a device. A current imbalance sensor is connected to the AC input and the floating ground conductor for generating an imbalance output signal in response to a ground fault in the aircraft electrical system.